In the case of a pump driven by an internal combustion engine, it is customary to control the drive speed of the engine according to the use condition of the pump. That is, for instance when hydraulic pressure is not required for a load connected to the pump, the discharge line of the pump is usually closed and the operating speed of the engine is kept as low as possible so as to save fuel as well as to reduce the noise of the engine. This is because quick discharge of the pump is usually required after the load is once put into inoperative condition and, thus, the engine is preferably kept driven even if the load is not operating. When the engine is kept driven, the discharged liquid from the pump may be returned to a drain tank or the like through a relief valve, etc. On the other hand, if the load is put into operation with hydraulic pressure, the hydraulic line to the load is fully opened whereby the engine speed is increased to its rated speed to meet the needed hydraulic pressure at the load. Such an automatic operating system for a pump driven by an internal combustion engine is disclosed, for example, in Japanese Utility Model Public Disclosure Nos. 127871/82 and 126584/82, etc.
However, an automatic operating system such as referred to above is provided with a pressure accumulator, a flow switch, an unloader valve, etc. in its discharging line arranged to receive an electric signal from the flow switch by a controller to control the drive speed of the engine. While the system above achieves its object of controlling the operation of the engine in response to the condition of the pump, such a system becomes expensive because it needs multiple instruments such as referred to above as well as an expensive controller and maintenance cost therefore becomes higher. Further, the possibility of failure in the instruments involved also becomes higher. In an apparatus operated under hydraulic pressure such as a high-pressure water washing machine or a hydraulic tester wherein a discharge line is repeatedly opened and closed to achieve the intended operation, it is preferable to keep the apparatus simple because a complex machine is more likely to break down.